Prime Minister's official spokesman reiterated that the prime suspects in the Salisbury attack were Russian spies

It comes after Moscow’s president said there is "nothing criminal" about Alexander Petrov and Ruslan Boshirov, who are said to be behind the attempted assassination of Sergei Skripal.

British authorities believe the pair smeared the highly toxic chemical Novichok on a door handle at the Wiltshire home of the ex-KGB officer.

Both Mr Skripal and his daughter Yulia were left critically ill, along with DS Nick Bailey, the policeman who attended to them.

The perfume bottle the members of Russian military intelligence service the GRU used to transport the Novichok was found three months later, killing Dawn Sturgess and hospitalising her partner Charlie Rowley.

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Detectives believe it is likely the two Salisbury suspects, thought to be aged around 40, travelled under aliases and that Mr Petrov and Mr Boshirov are not their real names.

Prosecutors deem it futile to apply to Russia for the extradition of the two men but a European Arrest Warrant has been obtained and the authorities are also seeking the assistance of Interpol.

Today the security minister Ben Wallace said they entered the UK from Russia via Gatwick Airport, but said requests for Russia to account for what happened in Salisbury had been met with "obfuscation and lies", saying their response merely "reinforces their guilt".